Surface Consumption Rate (SCR)
How much gas a diver breathes depends on the depth: at 20 meters every breath takes three times as much gas out of the tank as at the surface. The Surface Consumption Rate brings the gas used on a dive back to the pressure at the surface. It tells how many liters per minute a diver breathes, independent of depth and tank, so dives in different depths and with different tanks can be compared, and it is the value a gas plan starts from.
By Boyle's law the volume of a gas shrinks as the pressure rises. A breath at depth has the same volume in the lungs as at the surface, but it holds more gas: at an ambient pressure of 3 bar it holds three times as much. To compare consumption, the gas used is divided by the ambient pressure and by the time of the dive. The formula for calculating the SCR is:
SCR = (Pstart − Pend) × V / ((Depth / 10 + 1) × t)
- Pstart, Pend
- tank pressure at the beginning and at the end of the dive in bar
- V
- water volume of the tank in liters, e.g. 12 for a single 12 l
- Depth
- average depth of the dive in meters
- t
- time of the dive in minutes
Use the average depth, not the maximum depth. Most of a dive is spent shallower than its deepest point, and dividing by the pressure of the maximum depth makes the rate look lower than it is. A gas plan built on a rate that is too low runs out of gas. Many dive computers show the average depth in the log.
For instance, a diver breathes a 12 liter tank from 200 bar down to 100 bar in 20 minutes at an average depth of 20 meters. That is 1200 liters of gas at an ambient pressure of 3 bar, so the SCR is 1200 / (3 × 20) = 20 liters per minute.
The rate changes with workload, cold, stress and experience. A value from a relaxed dive is lower than one from a dive against a current, so plan with a rate from dives like the one ahead, and with a reserve.